HostStartup IWebHostBuilder.Configure Does Not Document Behavior

[EDIT by guardrex to add the metadata ... @aethercowboy, please use the **This page** feedback button and form to open issues on the repo, which will add the metadata and ping the author automatically.]

Followup documentation request for https://github.com/dotnet/aspnetcore/issues/30741

Microsoft.AspNetCore.Hosting.WebHostBuilderExtensions.Configure does not indicate that it will entirely override the behavior of Startup if it is called.

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#### Document Details

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* ID: 613ad64e-bc62-edf7-2e86-42210de718f4
* Version Independent ID: aabbbc32-3237-5ef9-c422-892dc6884a22
* Content: [Use hosting startup assemblies in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/platform-specific-configuration?view=aspnetcore-5.0)
* Content Source: [aspnetcore/fundamentals/host/platform-specific-configuration.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/fundamentals/host/platform-specific-configuration.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-fundamentals**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**